Spark-gap transmitters are known as the oldest transmitters and date from the 1880s. Illustrated in FIG. 14 is a conventional spark-gap transmitter 1400 comprising an induction coil 1450 having a primary coil and a secondary coil. The transmitter 1400 further comprises a spark gap 1420 connected in parallel with the secondary coil of the induction coil 1450. The transmitter 1400 further comprises a tuning coil 1460 connected to one or more Leyden jars 1410 in series. The tuning coil 1460 and the one or more Leyden jars 1410 are together connected in parallel with the spark gap 1420. The secondary coil of the induction coil 1450 and the one or more Leyden jars 1410 together form an LC resonator. The tuning coil 1480 is connected to group 1470 and to an antenna 1480.
The tuning circuit 1400 further comprises a telegraph key 1430 and one or more batteries 1440 connected in series. The telegraph key 1430 and the one or more batteries 1440 are together connected in parallel with the primary coil of the induction coil 1450. The telegraph key 1430 selectively couples and decouples the batteries 1440 from the primary coil of the induction coil 1450 to provide an instantaneous high-voltage pulse to the spark gap 1420. When a spark takes place across a narrow gap of the spark gap 1420, the spark energy will be released in the form of heat and electromagnetic radiation to transmit a wireless signal. The signal may be encoded with Morse code.
Illustrated in FIG. 15 is a conventional receiver 1500 comprising a head telephone receiver 1510, a crystal detector 1520, a variable condenser, and a two slider tuning coil 1560 comprising a coil, a first slider, and a second slider. The coil of the two slider tuning coil 1560 is connected to an antenna 1580. The first slider is connected to ground 1570, and the second slider is connected to one port of the variable condenser 1530. The other port of the variable condenser 1530 is connected to one port of the crystal detector 1520. The other port of the crystal detector 1520 is connected to the group 1570. The head telephone receiver 1510 is connected in parallel with the crystal detector 1520. The receiver 1500 receives the Morse code signal transmitted by the transmitter 1400.